Neutron Radiography

Introduction

Neutron radiography uses the unique interaction probabilities of neutrons to create images of materials. This imaging technique is non-destructive; however, samples receive significant radiation exposure. X-rays are attenuated best by materials with large atoms and not well by small, light ones. This allows x-rays to pass through the water in a human body easily and attenuate in bones. Conversely, neutrons move through high Z material easily and are attenuated well by low Z materials, such as water.

Much like x-rays neutrons can be used to create images of the internal characteristics of an object. Neutron radiography is typically used to create images of non-hydrogenous materials due to the fact that a neutron beam can pass through a significant amount of non-hydrogenous material without being completely attenuated. Materials made of heavier elements such as iron, silicon, or lead make good samples because changes in the thickness of these materials cause small but measurable changes in the neutron flux.

Experimental Setup

The neutron radiography system is located at beam port five of UT TRIGA reactor, located at the J.J. Pickle Research Campus. Beam port five is tangential to the reactor and it provides a high intensity, fast, collimated beam of neutrons. Samples are placed in front of a phosphor scintillation screen. Neutrons pass through the sample and strike the scintillation screen. Neutrons ionize the phosphorus in the screen, which cause it to produce flashes of light. The flashes of light are recorded by a camera and converted to numbers in a matrix. Figure 1 describes the configuration of the neutron radiography system.

Figure 1. Neutron radiography system.

The neutron source and collimator for these experiments are the TRIGA reactor and beam port five. The object in the figure above represents the sample that is to be imaged. The detector houses the scintillation screen, camera, and electronics to digitize the image and send it to the computer. The computer used in the radiography facility has a PCI card that allows it to receive the digitized images and record them. Labview is the software that is used to control the PCI card.

Longer exposure times increase the fidelity of the images. However, exposure times exceeding 10 milliseconds may lead to saturation, where entire sections of the image receive the maximum number of photons. In order to produce high fidelity images and circumvent saturation problems, several low quality 10 millisecond images are averaged together to create a single high quality image. Figure 2 shows the image quality difference between images produced from 1, 10, and 100 ten millisecond snapshots.

Figure 2. Images with 1 10 and 100, 10 millisecond snapshots.

Results

Several radiography images of four common metallic objects are presented below. Each image was produced by averaging 100 ten millisecond snapshots together. Radiography equipment is set up to generate several text files that each containing a matrix of numbers. The size of the numbers within the matrix is proportional to the quantity of photons emitted by the phosphorus screen at a given area. The text files are manipulated and averaged together within MATLAB to produce a jpeg image. Several different colormaps are available within MATLAB. The choice of colormap can emphasize different areas of an image. Several images are acquired before the sample is put in place so the presence of a significant flux gradient within the neutron beam can be negated. Images with and without background modifications are presented so one can judge the effect of the flux gradient within the beam on the unmodified image.